1. Field of the Invention
The invention relates to a regenerative heat exchanger and to a method for heating a gas in the regenerative heat exchanger The invention has particular applicability to the feeding of hot blast to a blast furnace in the iron making industry.
2. Description of the Related Art
Regenerative heat exchangers operate by passing a stream of a relatively hot gas through a heat exchange mass during one period (gas phase) to store heat in the mass. A stream of a relatively cool gas is subsequently passed in the reverse direction through the mass during a second period (blast phase) to recapture this stored heat. With heat exchangers of this type, it is customary to have the gas phase and the blast phase alternately recur, and to provide at least two heat exchange masses. In this way, while heat is being stored in one of the masses, heat can be recovered from the other mass. The refractory brick lined hot stove used in the iron making industry to feed blast furnaces with hot blast is one such example of a regenerative heat exchanger.
In some industries, such regenerative heat exchangers are referred to as hot stoves. Depending on the particular industry, multiple heat exchanger configurations may be preferred. For applications in which more than two heat exchangers of this kind are used, various phase setups may be implemented. Certain of these setups have been widely implemented in the industry with development having taken place over a long period of time. One such example is the refractory brick lined hot stove used in the iron making industry to feed blast furnaces with hot blast.
Problems associated with the conventional refractory brick lined regenerator are primarily inherent in the design of the regenerator itself. For example, these units are typically very tall and not compact. As a result of this large size, the cost of the units is very high.
The large size of the conventional regenerator can also lead to significant losses in system availability. In particular, when the operating pressure of the heat exchanger during the gas phase is lower than that during the blast phase, a pressurization period must be inserted after the gas phase and before the blast phase, and a depressurization period added after the blast phase and before the gas phase. During the depressurization phase, an amount of hot blast proportional to the unit volume is released into the atmosphere. This increases the heat losses of the regenerator by the heat quantity Q, according to the following equation:   Q  =                              C          P                ⁡                  (                                    T              Blast                        -                          T              Ref                                )                    ·              (                              P            Blast                    -                      P            Gas                          )            ·              V        Stove                    RT      Blast      
wherein:
Q is the heat loss during inversion phase (J)
Cp is the molecular heat capacity (J.molxe2x88x921.Kxe2x88x921)
TRef is the reference temperature (K)
TBlast is the blast temperature (K)
PBlast is the operating pressure during the blast phase (Pa)
PGas is the operating pressure during the gas phase. (Pa)
VStove is the free inner volume of the regenerator unit (m3), and
R is the ideal gas constant (8.314).
The periods for such phases, termed inversion phases, are longer with increased apparatus volumes. System availability is thus decreased as a result of the large size of the conventional systems.
In addition to reduced system availability during the inversion phases, further loss of availability results during system startup and shutdown. The refractory bricks, or checkers, lining the regenerator are typically constituted of a heat resistant masonry that is subject to thermal shock under high temperature variation over time. This particular design requires a very cautious and time consuming startup and shutdown. The time needed to start a new regenerator, i.e., to bring the temperature of the refractory lining to operating temperature, can be as long as one month. This period of time is required in order to safely dry the refractory masonry and to heat it up. The same caution must be applied to shutting down of the regenerator. To avoid deterioration of the refractory bricks of the regenerator, the cooling rate applied must stay within a given range depending on the nature of the refractory. These factors can significantly affect system availability.
In continuous processes, two or more regenerative heat exchangers are cyclically operated. The combination of the required inversion periods and the limitation on heating and cooling rates for the refractory checkers make it unrealistic, if not impossible, to use short cycle times (e.g., a two hour or less gas phase and a one hour or less blast phase). While modern equipment does allow for lessening of cycle times, practical limitations prevent the avoidance of inversion losses.
To overcome some of the disadvantages of conventional refractory lined hot stoves, regenerative heat exchangers of different geometrics have been proposed. One new design has drawn particular attention. Such regenerative heat exchangers are typically cylindrical in structure, and include a heat accumulation mass which consists of a loose bulk material arranged in a space and held in place between two concentric walls (i.e., an inner hot grid and an outer cold grid) which are permeable to gases. Regenerators of this type are disclosed, for example, in U.S. Pat. No. 2,272,108, U.S. Pat. No. 5,690,164 and U.S. Pat. No. 5,577,553. In the heat exchanger, a hot collection chamber is circumscribed by the inner hot grid for collecting the hot gases. A cold collection chamber for collecting the cooled gases is typically defined by the space between the outer cold grid and the external wall of the regenerator.
The quantitative embodiment described in U.S. Pat. No. 2,272,108, to Bradley, cannot operate in practice. The gas speed selected for passing through the heat accumulation mass is much too small while the size of the particles making up the loose bulk material of the heat accumulation mass is too large. This results in an inadequately small head loss of the gas in the material bed. The pressure of the gas thus decreases with height in the cold collection chamber. This effect, known as the xe2x80x9cstack effectxe2x80x9d, is negligible in the hot collection chamber. The pressure difference caused by the stack effect is a multiple of the pressure drop in the material bed. Consequently, when heating the regenerator, the heating gases flow only in the upper region through the material bed. Backflow of the gases might even be expected in the lower region. When working under hot blast, i.e., during cold blowing, the conditions are reversed. That is to say that only the lower region of the material bed would be exposed to the gases. These results lead to the conclusion that the regenerator described in this document would necessarily fail.
Further problems associated with heat exchanger design and the aforementioned stack effect concern the hot grid structures and their tendency to accumulate dust. As a result of dust accumulation, flow of the gas through the grid is inhibited during the blast and gas phases. This results in an increase in pressure drop through the brick and heat accumulation bed.
The main concern regarding dust loading of the gas stream is plugging of the openings of the bricks in the grid, as well as sticking of the particles in the heat accumulation bed. It has been found that particles in direct proximity to the hot grid openings tend to become coated by a hard, sintered layer of dust. This dust layer acts as a cement, binding the particles together in the regions close to the hot grid openings. As a result, the porosity of the heat accumulation bed becomes decreased, and the pressure drop through the bed increases. This phenomenon is particularly detrimental to the heat transfer efficiency of the heat exchanger.
Moreover, the high operating temperatures and thermal cycles experienced by the hot grid place extreme demands on that structure. In this respect, the succession of blast phase and gas phase cycles submits the hot grid to repeated stress cycles. The mechanical stress under which the bricks and hot grid can operate is generally limited by its weak point. Such a weak point typically occurs each time an important structural change in the brick occurs. The junction between the structures is often a potential crack development location.
U.S. Pat. No. 5,577,553, to Fassbinder discloses a hot grid made up of individual bricks composed of a heat resistant material, such as ceramic. The bricks have a cavity which opens into an annular chamber containing the heat-storage medium. The cavity is filled with pellets which are mutually consolidated and secured against dropping out of the brick by a heat resistant adhesive. A blind-hole bore, starting from the wall of the brick adjacent to the hot collecting chamber enclosed by the hot grid, extends into the cavity filled with the pellets. The disclosed brick, however, is disadvantageous in that its structure is complicated and is made of numerous pieces. The brick is thus more subject to stress build up and breakage is possible, especially at the junction between pellets and between pellets and brick. The adhesive material which glues the pellets together must withstand high stresses. Moreover, the production of such a brick is not easy and induces high costs.
It is an object of the present invention to provide a regenerative heat exchanger which avoids or conspicuously ameliorates the problems associated with the state of the art.
It is a further object of the invention to provide a method for heating a gas with the regenerative heat exchanger system.
The regenerative heat exchanger and method for heating a gas in accordance with the invention allow for shorter start up and shut down times, shorter cycle times and lower heat losses during inversion periods compared with conventional systems. The invention further results in lower cost for the unit as well as lower operational costs compared to conventional regenerators. Improved distribution of the gases passing through the heat accumulation bed is such that flow rate and other characteristics of the gas depend only on the radius of the point at which it is measured in the bed, and not on the height of the bed or the angle of flow.
Provided is a regenerative heat exchanger which features a chamber separated into a plurality of annular concentric spaces, comprising: a first, inner annular space defining a hot collection chamber; a second, outer annular space concentric to and around the first space defining a cold collection chamber; and a third annular space defining a heat exchange zone concentric to and between the first and second spaces. The heat exchange zone contains a particulate heat transfer material. The third space is supported on the inside by a concentrically disposed hot grid, and the external diameter of the third annular space is less than about double the internal diameter of the third annular space.
In accordance with a further aspect of the invention, a regenerative heat exchanger is provided which features a chamber separated into a plurality of annular concentric spaces, which include: a first, inner annular space defining a hot collection chamber; a second, outer annular space concentric to and around the first space defining a cold collection chamber; and a third annular space defining a heat exchange zone concentric to and between the first and second spaces. The heat exchange zone contains a particulate heat transfer material, wherein the third space is supported on the inside by a concentrically disposed hot grid. A combustion chamber is at least partially disposed within the hot collection chamber.
In accordance with further aspects of the invention, methods for heating a gas in the inventive regenerative heat exchangers are provided. The methods involve passing a hot gas from the first annular space through the hot grid and the third annular space, thereby heating the heat transfer material, and subsequently passing a gas to be heated from the second annular space through the third annular space and the hot grid into the first annular space, thereby heating the gas to be heated.